The ability to transport large amounts of critical data over a network in a reliable manner is of critical importance today. Applications such as streaming video, live audio, or teleconferencing all place high demands on networks such as the Internet. When a system carrying such data crashes, critical communications may be lost, revenues lost, productivity affected and users frustrated.
FIG. 1 shows an example of a prior art communication system. FIG. 1 shows a sender and a receiver connected to a network cloud. For purposes of this disclosure, the sender and receiver may be any standard electronic devices which desire to communicate through an electronic network such as the Internet, a Local Area Network (LAN), or a Wide Area Network (WAN).
To the end user, the operation of the system in FIG. 1 should be transparent and error free. For example, an end user (receiver) watching streaming video originating from a video server (sender) should never know what is taking place within the network cloud that makes the process possible.
FIG. 2 is a more detailed diagram of a prior art communications system. FIG. 2 expands on the detail of FIG. 1 by showing an example of communications occurring over a standard Internet connection. FIG. 2 includes a host and a server connected to a network cloud comprising a plurality of routers. In FIG. 2, the host wishes to transmit a packet P to the server. As is known by those of ordinary skill in the art, when the packet P arrives at router 1, the router 1 will encode the packet P with a unique identifier containing the source and destination addresses. Then router 1 will forward the packet P onto the destination through other routers according to standard routing protocols. In this example, router 1 will forward packet P onto router 4, which will then forward the packet P onto the ultimate destination, which in our example here is the server.
One application of the Internet that is seeing wider use in small and medium-sized businesses is Internet-based telephony. As the backbone of the Internet continues to be upgraded and expanded, the promise of a low-cost substitute to the traditional PBX system may now be realized.
One type of Internet-based telephony system that is gaining acceptance is IP telephony, which transfers voice information over the Internet Protocol (IP) of the TCP/IP protocol suite. While many standards exist, such as Voice over Packet (VOP) for Frame Relay and ATM networks, as used herein the term “IP telephony” will be used to designate voice over any packet-based network. In IP telephony, a user wishing to communicate uses an IP telephone, which is a device which transports voice over a network using data packets instead of the traditional switched circuits of a voice only network.
FIG. 3 shows an IP telephony system 100 of the prior art. System 100 includes a business system 102 configured to provide IP telephony in an enterprise environment. Business system 102 may include a network 104, such as a corporate Ethernet LAN, to which a plurality of IP telephones 106 may be operatively coupled to network 104 using hardware and software standard in the art. To couple the business system 102 to the outside world, typically a gateway 108 standard in the art is provided and operatively coupled between network 104 and backbone network 110.
Backbone network 110 may be any packet-based network standard in the art, such as IP, Frame Relay, or ATM. To provide voice communications to legacy POTS phones, typically a gateway 112 is provided, which may be a VoP gateway. Gateway 112 provides access to the Public Switched Telephone Network (PSTN) 114. Through PSTN 114, voice-only communications may be provided to legacy POTS phones 116.
The system 100 of FIG. 3 also includes an example of a broadband residential system 118. To reach individual residences, typically local ISP providers provide a cable or DSL head end 120 standard in the art. An individual wishing to utilize the ISP's service may then employ a cable modem or DSL modem 122 coupled to the user's home LAN 124. The user's home LAN may be provided by a home PC 126 configured to run software standard in the art such as Microsoft Windows®. The user may then operatively couple an IP telephone 128 to the LAN 124.
Thus, in the system 100 of FIG. 3, IP telephones 106 in business system 102 may communicate by voice with other similar business systems similarly configured with IP telephones. For a business enterprise, communication by IP telephony may be advantageous because the need for a traditional PBX system can be eliminated. Furthermore, an IP telephony system is scalable and may be upgraded along with the enterprise's network system.
Key to providing routers to small and medium-sized enterprises is manufacturing reliable routers at the lowest cost possible. Prior art routers provided to large enterprises are often complex in operation and expensive.
FIG. 4 is a conceptual block diagram of a typical prior art router. A typical prior art router may have a plurality of input ports and a plurality of output ports connected through a switching fabric which forms the heart of the router. Routers will typically contain a routing processor containing standard hardware and software, and may also contain a firewall application standard in the art as shown in FIG. 4.
In operation, when a packet appears at an input port such as input port 1 in FIG. 4, the firewall application will first examine the packet to see if the packet is safe to route through. If the packet is safe, then the routing processor will route the packet through the switching fabric to the appropriate output port, such as output port 1 as shown in FIG. 4.
As is known by those of ordinary skill in the art, the switching fabric contained within a high-end router comprises fast but expensive semiconductor switches which places such a router out of reach for smaller enterprises.
In order to satisfy smaller enterprises serving 20–50 people, lower cost routers have been developed which utilize software-based switches instead of a switching fabric.
FIG. 5 shows a conceptual diagram of a prior art software-based router. The router of FIG. 5 includes a central processor, such as a Motorola 860 Power PC. The router may be coupled to a LAN, such as an Ethernet-based network, and a WAN, such as a T1 line. The router may also be coupled to an IP cloud through either the LAN or WAN.
One or more POTS phones may also be coupled to the router through Foreign Exchange Station (FXS) ports 1 through n. The router may also include a digital signal processor (DSP) for interfacing the FXS ports with the processor. Typically, the DSP is configured to detect when a POTS phone is “off-hook”, or picked up from its cradle. Furthermore, the DSP is configured to detect and translate the DTMF tones produced when a caller dials a number on the phone.
Finally, the router of FIG. 5 may also include a power supply for providing all necessary power requirements for the router. Additionally, the power supply also must provide power to the POTS phones coupled to the router's FXS ports.
However, to produce low-cost routers, many design trade-offs must be made. One tradeoff is made in the selection of a power supply. The type and capacity of power supply chosen in software-based routers is of crucial importance to the final price of the router. Typically lower-cost switching power supplies are specified for software-based routers in order to keep costs down. Such power supplies often have a lower power capacity than the power supplies provided in high end routers. Power supply selection may be based upon the such considerations as thermal requirements, cost, and application demands.
However, specifying lower-capacity power supplies can present problems. In order to understand the problems associated with lower-capacity power supplies in routers, some background is necessary regarding traditional telephone operation.
FIG. 6 is a diagram showing representative waveforms present on the tip and ring connections of a POTS phone utilizing a balanced method for ringing. When a phone is in the On Hook state, typically the tip and ring show a constant voltage between −48 and −72 Volts. In the On Hook state, little or no current is drawn by the phone. When the phone is rung and placed in a ringing state, waveforms are placed on the tip and ring connections which oscillate +/−24 Volts about −48 Volts, and which are 180° out of phase. In the ringing state, the phone draws current which is provided by a chip called SLIC (Subscriber Line Interface Card). The SLIC has a finite capacity to drive current onto the line.
The amount of current drawn by a phone has been traditionally expressed as a Ringer Equivalence Number, or REN. One REN is defined as the amount of current passing through a resistive load of 7 k Ohm coupled between the tip and ring.
One of the demands on the router's power supply is the “ring capacity” which is how many telephones can be rung at once, or how many REN may be supported at any one time. In many low-cost routers on the market, 5 REN per line may be supported by the output section of a FXS port. However, it is possible to ring more lines than the power supply may handle, causing the power supply to overload, since many routers feature 16 lines per router.
When a switching power supply overloads, typically there is no warning and the power supply simply fails. If the power supply's capacity is reached and shuts down, the router will also shut down, losing the information concerning all connections being made through the router. This will appear to users of IP telephones as dropped calls, or calls which will not go through. In addition, other data connections will be dropped as well. Loss of transaction related data means the transaction has to be revalidated. Such problems are unsatisfactory to users.
Hence, there is a need for a method and apparatus for managing the ringing of FXS ports on software-based routers which will prevent power supply overload.